Can banging sound at speed = bad D/S?

On the highway at above 70 mph i get a slight banging sound and feel (feel it in the floor pan) under light acceleration, if I accelerate to say 85 mph and then coast back down to 70 it sounds / feels horrible.

DO i have an unbalanced D/S or possibly a bad u-joint? This only started after the 3.73's were put in but I didn't drive fast enough before the gear swap to have simmilar driveshaft RPMs. I only went 100+ mph a couple times and i "think" i remember this being there...

Help.

Chris

I know i have play in the splined output shaft of the trans without the D/S. I did jsut replace the outer seal recently.

I wouldn't say it is a bad vibration as in it is devinitely a vibration that i can hear nbut not one that I can feel. But this is likely due to where it happens. it sounds jsut like gear whine (since it happened after the bad gear install i bet it is gear whine).

I haven't replaced the trans mount ( I have the poly one from the kit) but i had checked it by pushing up and around onthe trans tail shaft and I noticed no movement at all.

Thanks for the thoughts...

Chris